Process liquid dispense method and apparatus

ABSTRACT

Methods of dispensing process liquid to provide a uniform layer of the liquid on a surface are disclosed in accordance with the present invention. The methods include dispensing a process liquid on the surface and rotating the surface at a first speed to distribute an effective amount of the process liquid to substantially wet the surface. The method further includes rotating the surface at a second speed to distribute an effective amount of the process liquid to produce a layer of the process liquid on the surface. In a preferred embodiment for dispensing photoresist onto the surface of a semiconductor wafer, the method includes rotating the wafer at the first speed prior to dispensing the photoresist onto the surface. The preferred method further includes decelerating the wafer from the first to the second speed during the dispensing of the photoresist and terminating the dispensing process after the second speed is reached.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/203,794,filed Dec. 1, 1998, which is a continuation of application Ser. No.08/909,572, filed Aug. 12, 1997, now U.S. Pat. No. 5,912,049.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention generally relates to dispensing liquids onto asurface. More particularly, the present invention relates to methods ofspin dispensing process liquids, such as photoresist, onto a surface ofa semiconductor wafer that more evenly distribute the process liquidsover the surface and reduce the number of defects resulting frommaldistribution of the liquid on the surface.

Integrated circuits are solid state devices in which electricalcomponents and electrical connections between the components areincorporated into a solid matrix. The circuits are formed by thestrategic placement of various conducting, semiconducting and insulatingmaterials on a substrate. The development of the integrated circuit hasled to the miniaturization of electronics by providing a strong matrixto support and protect fragile miniaturized components and connectionsand by facilitating the placement of the electrical components in closeproximity. The integrated circuit has further served to increase thereliability of electronic devices by the elimination of moving parts andfragile electrical wiring and connections.

Integrated circuits are typically constructed by depositing a series ofindividual layers of predetermined materials on a wafer shapedsemiconductor substrate, or “wafer”. The individual layers of theintegrated circuit are in turn produced by a series of manufacturingsteps. For example, silicon dioxide is typically deposited over apreviously formed circuit layer to provide an insulating layer for thecircuit. Subsequent circuit layers are formed on the wafer using aradiation alterable material, known as photoresist.

Photoresist materials are generally composed of a mixture of organicresins, sensitizers and solvents. Sensitizers are compounds, such asdiazonaphthaquinones, that undergo a chemical change upon exposure toradiant energy, such as visible and ultraviolet light. The irradiatedsensitizer generally has different physical properties than thenon-irradiated sensitizer, such as differing solvation characteristics.Resins are used to provide mechanical strength to the photoresist.Solvents are added to lower the viscosity of the photoresist, whichfacilitates a more uniform application of the liquid over the surface ofthe wafers.

A smooth, level layer of photoresist is formed on the wafer to provide aproper surface for depositing additional layers in the production of thecircuit. After the photoresist layer is formed, it is typically heatedto evaporate the solvents and harden the layer.

The hardened photoresist layer is then selectively irradiated to producea layer having varying salvation characheristics. A radiation maskhaving transparent and opaque portions that define the next circuitlayer pattern is used in conjunction with a radiation source toselectively expose the layer to radiation.

Following irradiation, the photoresist layer is exposed to a chemical,known as developer, in which either the irradiated or the nonirradiatedphotoresist is soluble. The soluble portion of the photoresist isdissolved exposing portions of the underlying insulating layer in thepattern defined by the mask. Developer solutions need to be uniformlydistributed over the substrate surface to facilitate uniform dissolutionof the photoresist layer.

Photoresist and developer solutions and other process liquids aretypically applied to the wafer while the wafer is being spun on arotating chuck, using a technique known as a spin dispensing, orcoating. The liquid may be dispensed before the wafer is spun (i.e.,statically) or while the wafer is spinning (i.e., dynamically). Thespinning of the wafer distributes the liquid over the surface of thematerial.

The final thickness and uniformity of a process liquid layer depends ona number of variables. Spin variable, such as spinning speed, time andacceleration, dispense techniques, times and quantities can greatlyaffect the layer. The system pressure, temperature, and exhaust flowrate, as well as the physical properties of the process liquid, such asthe viscosity and the vapor pressure of the volatile components, alsoaffect the thickness and uniformity. If the spin variables are notmatched to the process liquid, the resulting layer may contain anunacceptable number of defects. Commonly, a string of defects occursstarting near the center of the surface and extending radiallyperpendicular to the edge of the surface. The strings of defects, calledstriations, are thought to be air bubbles trapped in the liquid duringthe coating process.

Generally, efforts to provide a more uniform layer of process liquidduring spin dispensing have focused on either changing the dispensenozzle design to provide a different dispense pattern and varying thespin pattern of the substrate to alter the distribution pattern of theliquid after it has been dispensed onto the surface of the substrate.Examples of the different nozzles used to dispense process liquid can befound in U.S. Pat. No. 5,002,008 issued to Ushijima et al., U.S. Pat.No. 5,020,200 issued to Mimasaka et al. and U.S. Pat. No. 5,429,912issued to Neoh to name a few.

A number of spin techniques have been developed attempting to uniformlydistribute the process liquid over the surface of the substrate. Forexample, U.S. Pat. No. 4,741,926 issued to White discloses spin coatingan organic material at a speed of not less than 4000 rpm, preferably6000-8000 rpm, until a build up of coating is detectable on a side wallof a topographical feature on the surface. The rotational speed is thendecelerated to less than 4000 rpm, preferably 1000-3500 rpm to produce alayer of desired thickness. A difficulty with the White process is thata sensor must be positioned to detect the build up of coating materialon the side wall. Because the location of the build up will most likelyvary from wafer to wafer, it may be difficult to reliably orconsistently detect. Also, an excess amount of coating material may benecessary to compensate for material spun off the wafer before the buildup of coating material is detected on the side wall.

In U.S. Pat. No. 5,405,813 issued to Rodrigues, methods are disclosedinvolving four different spinning steps for optimizing the distributionof photoresist on a semiconductor wafer. The methods provide forspinning the wafer at a first rotational speed and then decelerating thewafer. Photoresist is applied during the deceleration until a secondrotational speed is reached and the dispensing of the photoresist isstopped. The wafer is then accelerated to a third rotational speed toproduce a layer of a desired thickness and further accelerated to afourth rotational speed to dry the coating layer.

In U.S. Pat. No. 5,453,406 (the '406 patent) issued to Chen, methods aredisclosed for providing an aspect ratio independent spin on glass (SOG)coating by dispensing a first layer of the coating at a low speedfollowed by second layer of the coating dispensed at a high speed. Thismethod requires that one additional step be added to the productionprocess for each coating step in the prior art processes.

U.S. Pat. No. 5,567,660 issued to Chen et al. discloses a processsimilar to the '406 patent. A SOG is statically dispensed on thesubstrate and the substrate is spun at a first low speed followed by asecond higher speed. A difficulty encountered in this method, as withother methods employing lower spin speeds, is that radial striations mayoccur in the coating layer. Radial striations are generally a result ofeither the dispense liquid not having sufficient momentum to bedistributed evenly over a nonplanar surface or the liquid notpreferentially wetting the substrate surface.

U.S. Pat. No. 5,066,616 issued to Gordon discloses methods for producinga layer of photoresist. The Gordon methods involve dispensing a liquidsolvent underlayer to initially wet the surface of the wafer. Aphotoresist layer is then spun on top of the solvent layer as a means tobetter ensure more complete coverage of the wafer surface by thephotoresist. The Gordon methods provide a solution of the problem ofmottling; however, the methods require the additional dispensing of asolvent layer prior to the photoresist layer. The solvent layer mayincrease the possibility of defect formation following the evaporationof the solvent underlayer and nonuniform thinning of the photoresist bythe solvent underlayer.

Thus, it is apparent that a need exists for an improved method for spindispensing process liquids, such as photoresist, which overcomes, amongothers, the above-discussed problems to produce a more uniform layer ofprocess liquid over the surface of the wafer.

BRIEF SUMMARY OF THE INVENTION

The above need and others are addressed by methods and apparatuses ofthe present invention that provide for increased uniformity in thedistribution of process liquid over a surface, namely photoresist over asemiconductor wafer substrate surface. The methods include dispensing aprocess liquid on the surface and rotating the surface at a first speedto distribute an effective amount of the process liquid to substantiallywet the surface. The method further includes rotating the surface at asecond speed to distribute an effective amount of the process liquid toproduce a layer of the process liquid having a desired thickness on thesurface.

In a preferred embodiment for dispensing photoresist onto the surface ofa semiconductor wafer, the method includes rotating the wafer at thefirst speed prior to dispensing the photoresist onto the surface. Thepreferred method further includes decelerating the wafer from the firstto the second speed while dispensing the photoresist, and terminatingthe dispensing process after spinning at the second speed for apredetermined period of time.

The present invention is generally applicable to process liquids havingboth high and low viscosities, as well as liquids having varyingaffinities for the surface. The methods of the present invention aremost applicable for distributing high viscosity and/or low affinityprocess liquids, such as photoresist, over a surface, such as asemiconductor substrate. Rotation of the surface at a high first speedprovides for a substantially wetted surface by imparting sufficientmomentum to flow the process liquid over nonplanar surface topographyand to overcome surface tension effects between the liquid and thesurface. The surface can then be decelerated to the second rotationalspeed that is selected to produce a desired final thickness of theprocess liquid layer.

Accordingly, the present invention provides methods for uniformlydistributing process liquid on a rotating surface. The present inventionmay also provide for less process liquid and slower rotational speeds toensure full coverage of the surface. These and other details, objects,and advantages of the invention will become apparent as the followingdetailed description of the present preferred embodiments thereofproceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described ingreater detail with reference to the accompanying drawings, wherein likemembers bear like reference numerals and wherein:

FIG. 1 is side view of a spin dispensing apparatus of the presentinvention; and,

FIG. 2 is a time line of the processing steps in a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Methods of the present invention will be described generally withreference to a spin dispensing, or coating, apparatus 10 shown in theFIG. 1 for the purpose of illustrating present preferred embodiments ofthe invention only and not for purposes of limiting the same. Theapparatus 10 includes a rotatable spin chuck 12 and a dispense nozzle 14directed toward the chuck 12. The nozzle 14 is used to dispense aprocess liquid, such as photoresist, onto a surface 16 of a substrate 18positioned on the chuck 12. Preferably, the apparatus 10 includes acontroller 20 connected to the dispense nozzle 14 and the chuck 12. Thecontroller 20 is used to control the dispensing of the process liquidand the rotation of the chuck 12 in accordance with the methods of thepresent invention. The controller 20 can be incorporated into spincoating apparatuses known in the art, such as an SVG 8800 coater trackor a TEL Mark 8, as well as other apparatuses that can be operated inaccordance with the methods of the present invention.

The methods provide for a more evenly distributed layer of processliquid over a substrate by imparting to the process liquid sufficientmomentum to overcome nonplanar topography on the surface and surfacetension forces in the liquid with respect to the surface. The methodscan generally be used to distribute any process liquid over the surfaceof the substrate. However, the methods are of greater utility when theprocess liquid has a high viscosity or does not preferentially wet thesurface (i.e., the process liquid has a low affinity for the surface).In particular, the methods are useful for distributing photoresist overthe surface of a semiconductor wafer. Photoresist compositions aregenerally high viscosity materials, such as polymethylmethacrylate(PMMA), novolac resins and polyimides, that have varying affinities forthe different surface layers placed on a substrate during the productionof an integrated circuit.

In a preferred embodiment of the method shown in FIG. 2, the substrate18 is placed on the chuck 12 and the chuck 12 and substrate 18 arerotated at a first speed. The first speed is chosen to impart sufficientmomentum to the process liquid dispensed onto the surface 16 of thesubstrate 18 that the process liquid will flow over nonplanar surfacetopography encountered and will overcome surface tension effects tosubstantially cover the entire surface 16. The selection of the firstspeed is not limited to a speed that will result in a layer of a giventhickness, but only to those speeds that will substantially wet thesurface 16. For example, a first speed of at least 4000 rpm wouldtypically impart sufficient momentum to distribute an effective amountof photoresist material to substantially wet a semiconductor surface 16.An additional benefit of rotating the surface 16 at a high rate is thatparticulate matter that may be on the surface 16 as contaminants may beremoved by the high velocity process liquid.

In a preferred embodiment, dynamic dispensing of process liquid to wetthe surface is performed with the surface rotating at the first speed,shown schematically at time t₁ in FIG. 2. Dynamic dispensing at thefirst spin speed immediately imparts sufficient momentum to the processliquid to distribute the liquid over the surface, thereby avoidingproblems that occur at low spin speeds. Static dispensing of the processliquid on the surface can be performed followed by rotationalacceleration of the surface to the first speed. However, unless thesurface is rapidly accelerated, striations and incomplete coverage ofthe wafer, similar to defects that occur at low spin speeds, may beencountered using the static dispense and acceleration methods.

Also in a preferred embodiment, the surface is decelerated to a secondspeed, when an effective amount of process liquid has been dispensed tosubstantially cover the entire surface. The process liquid is preferablydispensed during deceleration and terminated after the second speed isreached. The second speed is selected to distribute the process liquidover the surface to achieve a desired final thickness of the processliquid layer on the surface. Preferably, as shown by time t₂ in FIG. 2,dispensing of the process liquid is continued until after the secondspin speed is reached. An effective amount of the process liquid isdispensed during that time to produce the desired thickness layer on thesurface.

The sequencing of the process steps is performed to facilitate auniformly distributed layer in a number of ways. Dispensing of theprocess liquid during the deceleration of the surface is performed 1) todistribute additional process liquid over the surface of the wafernecessary to produce a layer of the desired final thickness; and 2) toprevent the surface from drying during deceleration. In addition,dispensing the photoresist during deceleration can reduce the total spintime required to distribute the process liquid to the periphery withoutrequiring an excess amount of process liquid.

Alternatively, the dispensing step can be terminated prior to or duringthe deceleration of the surface and reinitiated at the second speed.Depending upon the process liquid used, the wetting of the surface atthe first spin speed may increase the affinity of the surface for theprocess liquid. In that case, the drying of the process liquid may notgreatly affect or could potentially enhance the ability to uniformlydistribute the process liquid at a later step. Also, the dispensing canbe performed during deceleration, but terminated prior to achieving thesecond spin speed.

It is preferred that the spinning of the surface is continued at thesecond speed following the termination of the dispensing step. Theadditional spinning at the second speed serves to uniformly distributethe process liquid over the surface and/or to form a planar layer of theprocess liquid on the surface. When the distribution of the processliquid over the surface is complete, rotation of the surface is ceased.

In the alternative, additional steps can be performed following thespinning of the surface at the second speed. The surface can be furtherdecelerated and rotated to dry the process liquid on the surface and tolessen the possibility that additional spinning will adversely affectthe final layer thickness. The surface can also be accelerated toachieve a thinner layer by spinning additional process liquid off thesurface or to accelerate the drying of the process liquid. Thedispensing of process liquid may be restarted either at the second spinspeed or at different speeds to provide additional layers of the processliquid. The dispense step can be continued at the second spin speed toeffect a distribution pattern of the process liquid. The surface canalso be subjected to additional spinning steps, either fast or slower,to vary the characteristics within the layer.

An example of a preferred embodiment of the method is provided withrespect to producing a photoresist layer on a surface of a semiconductorsubstrate. The semiconductor wafer is placed on the spin chuck androtated at a first speed of at approximately 5000 rpm. Photoresistcompositions having a viscosity of approximately 20 centipoise or lesshave been found to be effectively distributed over the surface of an 8inch semiconductor wafer at these speeds. Photoresist at a temperatureof 21.5° C. is dispensed at 3 cm³/sec. from the dispense nozzle in a 23°C. spin coating environment for approximately 0.75 seconds, at whichtime an effective amount of photoresist had been dispensed to wet thesurface.

The surface is immediately decelerated at approximately 5000 rpm/sec.for approximately 0.5 sec to a second spin speed of approximately 2500rpm/sec. during which time dispensing of the photoresist continues.Dispensing of the photoresist is continued for an additional 0.75 sec.after the second speed is reached to dispense an effective amount of thephotoresist to produce a layer having the desired final thickness. Afterdispensing is terminated, spinning of the substrate is continued todistribute the photoresist over the surface and form a planar layer ofphotoresist on the surface. The rotation of the substrate is ceasedafter the photoresist is distributed and the substrate is removed fromthe spin chuck.

Those of ordinary skill in the art will appreciate that the presentinvention provides significant advantages over the prior art. Inparticular, the subject invention provides a method to reduce the numberof defects in coating layers by more uniformly distributing a processliquid over a surface. While the subject invention provides these andother advantages over prior art, it will be understood, however, thatvarious changes in the details, materials and arrangements of stepswhich have been herein described and illustrated in order to explain thenature of the invention may be made by those skilled in the art withinthe principle and scope of the invention as expressed in the appendedclaims.

What is claimed is:
 1. A method of producing a layer of a process liquidon a surface, comprising: rotating said surface at a first speed;decelerating said surface to a second speed, wherein said second speedis sufficient to produce a desired final thickness of said layer ofprocess liquid on said surface; and dispensing said process liquid onsaid surface during said deceleration for an appreciable time and in aneffective amount sufficient to produce a layer of desired finalthickness on said surface.
 2. The method of claim 1 wherein said surfaceis a semiconductor surface.
 3. The method of claim 1 wherein saidrotating includes rotating at a first speed sufficient to impartmomentum to flow said process liquid over nonplanar topography on saidsurface.
 4. The method of claim 1 wherein said rotating includesrotating at a first speed sufficient to impart momentum to said processliquid to overcome surface tension between said process liquid and saidsurface.
 5. The method of claim 1 wherein said rotating includesrotating at a first speed sufficient to distribute an effective amountof said process liquid to substantially wet said surface.
 6. The methodof claim 1 wherein said first speed is at least 4000 rpm.
 7. The methodof claim 6 wherein said first speed is approximately 5000 rpm.
 8. Themethod of claim 1 wherein said dispensing occurs for approximately 0.75seconds at a rate of approximately 3 cm³/sec.
 9. The method of claim 1wherein said decelerating occurs for approximately 0.5 seconds at a rateof approximately 2500 rpm/sec.
 10. The method of claim 1 furthercomprising terminating said dispensing of said process liquid.
 11. Themethod of claim 10 wherein said terminating dispensing occurs duringsaid deceleration of said surface from said first speed to said secondspeed.
 12. The method of claim 1 further comprising rotating saidsurface at a third speed.
 13. The method of claim 1 further comprisingceasing rotation of said surface.
 14. A method of producing a layer of aprocess liquid on a surface, comprising: rotating said surface at afirst speed; decelerating said surface to a second speed, wherein saidsecond speed is sufficient to produce a desired final thickness of saidlayer of process liquid on said surface; dispensing said process liquidon said surface during said deceleration for an appreciable time and inan effective amount sufficient to produce a layer of desired finalthickness on said surface; and terminating said dispensing of saidprocess liquid.
 15. The method of claim 14 wherein said surface is asemiconductor surface.
 16. The method of claim 14 wherein said firstspeed is at least 4000 rpm.
 17. The method of claim 14 furthercomprising rotating said surface at a third speed.
 18. The method ofclaim 14 further comprising ceasing rotation of said surface.
 19. Amethod of producing a layer of a process liquid on a surface,comprising: dispensing said process liquid on said surface; rotatingsaid surface at a first speed to distribute an effective amount of saidprocess liquid to substantially wet said surface; decelerating saidsurface from said first speed to a second speed during said dispensing,said dispensing occurring during deceleration for an appreciable timeand in an effective amount sufficient to produce a layer of desiredthickness on said surface, said second speed selected to produce a layerof desired final thickness on said surface; and rotating said surface atsaid second speed to distribute an effective amount of said processliquid to produce a layer of said process liquid on said surface.
 20. Amethod of producing a layer of a photoresist on a surface, comprising:dispensing said photoresist on said surface; rotating said surface at afirst speed to distribute an effective amount of said photoresist tosubstantially wet said surface; decelerating said surface to a secondspeed during said dispensing, said dispensing occurring duringdeceleration for an appreciable time and in an effective amountsufficient to produce a layer of desired final thickness on saidsurface, said second speed selected to produce a layer of desired finalthickness on said surface.
 21. A method of producing a layer of aphotoresist on a surface, comprising: dispensing said photoresist onsaid surface; rotating said surface at a first speed; and deceleratingsaid surface to a second speed during said dispensing, said dispensingoccurring during deceleration for an appreciable time and in aneffective amount sufficient to produce a layer of desired finalthickness on said surface, said second speed selected to produce a layerof desired final thickness on said surface.
 22. A method of producing alayer of a photoresist on a surface, comprising: rotating said surfaceat a first speed; dispensing said photoresist on said surface duringsaid rotation; and decelerating said surface to a second speed duringsaid dispensing, said dispensing occurring during deceleration for anappreciable time and in an effective amount sufficient to produce alayer of desired final thickness on said surface, said second speedselected to produce a layer of desired final thickness on said surface.